Density of states in graphene with vacancies: midgap power law and frozen multifractality

The density of states (DoS), $\varrho(E)$, of graphene is investigated numerically and within the self-consistent T-matrix approximation (SCTMA) in the presence of vacancies within the tight binding model. The focus is on compensated disorder, where the concentration of vacancies, $n_\text{A}$ and $n_\text{B}$, in both sub-lattices is the same. Formally, this model belongs to the chiral symmetry class BDI. The prediction of the non-linear sigma-model for this class is a Gade-type singularity $\varrho(E) \sim |E|^{-1}\exp(-|\log(E)|^{-1/x})$. Our numerical data is compatible with this result in a preasymptotic regime that gives way, however, at even lower energies to $\varrho(E)\sim E^{-1}|\log(E)|^{-\mathfrak{x}}$, $1\leq \mathfrak{x} < 2$. We take this finding as an evidence that similar to the case of dirty d-wave superconductors, also generic bipartite random hopping models may exhibit unconventional (strong-coupling) fixed points for certain kinds of randomly placed scatterers if these are strong enough. Our research suggests that graphene with (effective) vacancy disorder is a physical representative of such systems.Comment: References updated onl

Detection of a period decrease in NN Ser with ULTRACAM: evidence for strong magnetic braking or an unseen companion?

We present results of high time resolution photometry of the eclipsing pre-cataclysmic variable NN Ser. We observed 13 primary eclipses of NN Ser using the high-speed CCD camera ULTRACAM and derived times of mid-eclipse, from fitting of light curve models, with uncertainties as low as 0.06 s. The observed rates of period change appear difficult to reconcile with any models of orbital period change. If the observed period change reflects an angular momentum loss, the average loss rate is consistent with the loss rates (via magnetic stellar wind braking) used in standard models of close binary evolution, which were derived from observations of much more massive cool stars. Observations of low-mass stars such as NN Ser's secondary predict rates of ~100 times lower than we observe. We show that magnetic activity-driven changes in the quadrupole moment of the secondary star (Applegate, 1992) fail to explain the period change by an order of magnitude on energetic grounds, but that a light travel time effect caused by the presence of a third body in a long (~ decades) orbit around the binary could account for the observed changes in the timings of NN Ser's mid-eclipses. We conclude that we have either observed a genuine angular momentum loss for NN Ser, in which case our observations pose serious difficulties for the theory of close binary evolution, or we have detected a previously unseen low-mass companion to the binary.Comment: 10 pages, 6 figures. Accepted for publication in MNRA

Precise mass and radius values for the white dwarf and low mass M dwarf in the pre-cataclysmic binary NN Serpentis

We derive precise system parameters for the pre-cataclysmic binary, NN Ser. From light curve fitting we find an orbital inclination of i = 89.6 +/- 0.2 deg. From the HeII absorption line we find K_{WD}= 62.3 +/- 1.9 km/s. The irradiation-induced emission lines from the surface of the secondary star give a range of observed radial velocities. The corrected values give a radial velocity of K_{sec}= 301 +/- 3 km/s, with an error dominated by the systematic effects of the model. This leads to a binary separation of a = 0.934 +/- 0.009 R_{sun}, radii of R_{WD} = 0.0211 +/- 0.0002 R_{sun} and R_{sec} = 0.149 +/- 0.002 R_{sun} and masses of M_{WD} = 0.535 +/- 0.012 M_{sun} and M_{sec} = 0.111 +/- 0.004 M_{sun}. The masses and radii of both components of NN Ser were measured independently of any mass-radius relation. For the white dwarf, the measured mass, radius and temperature show excellent agreement with a `thick' hydrogen layer of fractional mass M_{H}/{M}_{WD} = 10^{-4}. The measured radius of the secondary star is 10% larger than predicted by models, however, correcting for irradiation accounts for most of this inconsistency, hence the secondary star in NN Ser is one of the first precisely measured very low mass objects to show good agreement with models. ULTRACAM r', i' and z' photometry taken during the primary eclipse determines the colours of the secondary star as (r'-i')_{sec}= 1.4 +/- 0.1 and (i'-z')_{sec} = 0.8 +/- 0.1 which corresponds to a spectral type of M4 +/- 0.5. This is consistent with the derived mass, demonstrating that there is no detectable heating of the unirradiated face, despite intercepting radiative energy from the white dwarf which exceeds its own luminosity by over a factor of 20.Comment: 20 pages, 17 figures, 8 tables, minor changes, accepted for publication in MNRA

The planets around NN Serpentis : still there

We present 25 new eclipse times of the white dwarf binary NN Ser taken with the high-speed camera ULTRACAM on the William Herschel Telescope and New Technology Telescope, the RISE camera on the Liverpool Telescope and HAWK-I on the Very Large Telescope to test the two-planet model proposed to explain variations in its eclipse times measured over the last 25 yr. The planetary model survives the test with flying colours, correctly predicting a progressive lag in eclipse times of 36 s that has set in since 2010 compared to the previous 8 yr of precise times. Allowing both orbits to be eccentric, we find orbital periods of 7.9 ± 0.5 and 15.3 ± 0.3 yr, and masses of 2.3 ± 0.5 and 7.3 ± 0.3 MJ. We also find dynamically long-lived orbits consistent with the data, associated with 2:1 and 5:2 period ratios. The data scatter by 0.07 s relative to the best-fitting model, by some margin the most precise of any of the proposed eclipsing compact object planet hosts. Despite the high precision, degeneracy in the orbit fits prevents a significant measurement of a period change of the binary and of N-body effects. Finally, we point out a major flaw with a previous dynamical stability analysis of NN Ser, and by extension, with a number of analyses of similar systems

Results from the Commissioning of the n-TOF Spallation Neutron Source at CERN

The new neutron time-of-flight facility (n_TOF) has been built at CERN and is now operational. The facility is intended for the measurement of neutron induced cross sections of relevance to Accelerator Driven Systems (ADS) and to fundamental physics. Neutrons are produced by spallation of the 20 GeV/c proton beam, delivered by the Proton Synchrotron (PS), on a massive target of pure lead. A measuring station is placed at about 185 m from the neutron producing target, allowing high-resolution measurements. The facility was successfully commissioned with two campaigns of measurements, in Nov. 2000 and Apr. 2001. The main interest was concentrated in the physical parameters of the installation (neutron flux and resolution function), along with the target behavior and various safety-related aspects. These measurements confirmed the expectations from Monte Carlo simulations of the facility, thus allowing to initiate the foreseen physics program

3D Gamma-ray and Neutron Mapping in Real-Time with the Localization and Mapping Platform from Unmanned Aerial Systems and Man-Portable Configurations

Nuclear Scene Data Fusion (SDF), implemented in the Localization and Mapping Platform (LAMP) fuses three-dimensional (3D), real-time volumetric reconstructions of radiation sources with contextual information (e.g. LIDAR, camera, etc.) derived from the environment around the detector system. This information, particularly when obtained in real time, may be transformative for applications, including directed search for lost or stolen sources, consequence management after the release of radioactive materials, or contamination avoidance in security-related or emergency response scenarios. 3D reconstructions enabled by SDF localize contamination or hotspots to specific areas or objects, providing higher resolution over larger areas than conventional 2D approaches, and enabling more efficient planning and response, particularly in complex 3D environments. In this work, we present the expansion of these gamma-ray mapping concepts to neutron source localization. Here we integrate LAMP with a custom $Cs_2LiLa(Br,Cl)_6:Ce$ (CLLBC) scintillator detector sensitive to both gamma-rays and neutrons, which we dub Neutron Gamma LAMP (NG-LAMP). NG-LAMP enables simultaneous neutron and gamma-ray mapping with high resolution gamma-ray spectroscopy. We demonstrate the ability to detect and localize surrogate Special Nuclear Materials (SNM) in real-time and in 3D based on neutron signatures alone, which is critical for the detection of heavily shielded SNM, when gamma-ray signatures are attenuated. In this work, we show for the first time the ability to localize, in 3D and realtime, a neutron source in the presence of a strong gamma-ray source, simultaneous and spectroscopic localization of three gamma-ray sources and a neutron source, and finally the localization of a surrogate SNM source based on neutron signatures alone, where gamma-ray data are consistent with background

DE Canum Venaticorum : a bright, eclipsing red dwarf–white dwarf binary

Context. Close white dwarf–red dwarf binaries must have gone through a common-envelope phase during their evolution. DE CVn is a detached white dwarf–red dwarf binary with a relatively short (∼8.7 h) orbital period. Its brightness and the presence of eclipses makes this system ideal for a more detailed study. Aims. From a study of photometric and spectroscopic observations of DE CVn we derive the system parameters that we discuss in the framework of common-envelope evolution. Methods. Photometric observations of the eclipses are used to determine an accurate ephemeris. From a model fit to an average lowresolution spectrum of DE CVn, we constrain the temperature of the white dwarf and the spectral type of the red dwarf. The eclipse light curve is analysed and combined with the radial velocity curve of the red dwarf determined from time-resolved spectroscopy to derive constraints on the inclination and the masses of the components in the system. Results. The derived ephemeris is HJDmin = 2 452 784.5533(1) + 0.3641394(2) × E. The red dwarf in DE CVn has a spectral type of M3V and the white dwarf has an effective temperature of 8 000 K. The inclination of the system is 86+3◦ −2 and the mass and radius of the red dwarf are 0.41 ± 0.06 M and 0.37+0.06 −0.007 R, respectively, and the mass and radius of the white dwarf are 0.51+0.06 −0.02 M and 0.0136+0.0008 −0.0002 R, respectively. Conclusions. We found that the white dwarf has a hydrogen-rich atmosphere (DA-type). Given that DE CVn has experienced a common-envelope phase, we can reconstruct its evolution and we find that the progenitor of the white dwarf was a relatively lowmass star (M ≤ 1.6 M). The current age of this system is 3.3−7.3 × 109 years, while it will take longer than the Hubble time for DE CVn to evolve into a semi-detached system

Interparticle interactions:Energy potentials, energy transfer, and nanoscale mechanical motion in response to optical radiation

In the interactions between particles of material with slightly different electronic levels, unusually large shifts in the pair potential can result from photoexcitation, and on subsequent electronic excitation transfer. To elicit these phenomena, it is necessary to understand the fundamental differences between a variety of optical properties deriving from dispersion interactions, and processes such as resonance energy transfer that occur under laser irradiance. This helps dispel some confusion in the recent literature. By developing and interpreting the theory at a deeper level, one can anticipate that in suitable systems, light absorption and energy transfer will be accompanied by significant displacements in interparticle separation, leading to nanoscale mechanical motion